VOLUME instruction and volume mounts

[nuest]

• [ ] implement VOLUME statement, see https://docs.docker.com/engine/reference/builder/#volume
• [ ] add example in vignette on how to access a volume created automatically by Docker based on a VOLUME statement (without using -v), i.e. with docker inspect (see also https://stackoverflow.com/questions/40163036/difference-between-volume-declaration-in-dockerfile-and-v-as-docker-run-paramet)
• [ ] add example to vignette with
  • volume mounting using Docker on CLI
  • harbor (plain R)